1. Field of the Invention
The present invention relates to mine roof bolts and, more particularly, to tensionable cable bolts having a mixing delay device.
2. Brief Description of the Prior Art
Mine roofs are often supported by rock bolts, cable bolts, trusses, and bearing plates. A rock bolt generally is a solid longitudinally extending rod, such as concrete reinforcement bar, having a drive head integrally formed or otherwise attached to a first end of the rod. In contrast, a cable bolt typically is a multi-strand cable segment with a drive head attached to a first end of the cable segment by welding, swaging, or other suitable method. Either of these types of mine roof bolts may be tensionable or non-tensionable, with tensionable rock or cable bolts generally including a mechanical anchor. U.S. Pat. No. 4,419,805 to Calandra, Jr., assigned to the applicant of the present invention and herein incorporated by reference in its entirety, discloses a tensionable rock bolt. U.S. Pat. No. 6,074,134 to Stankus et al., assigned to the applicant of the present invention and herein incorporated by reference in its entirety, discloses a tensionable cable bolt.
Because tensionable cable bolts are less rigid than tensionable rock bolts, tensionable cable bolts are more likely to bend without breaking if rock strata above the mine roof shifts after installation of the tensionable cable bolt. However, one drawback of known tensionable cable bolts is torsional deformation when torque is applied to a drive head positioned adjacent to a first end of the tensionable cable bolt. When torque is applied to the drive head, a mechanical anchor and/or resin positioned between the first end and a second end of the tensionable cable bolt restrains rotational movement of the cable bolt while the first end of the tensionable cable bolt is left unencumbered. Continued rotation at the first end tends to cause twisting of the tensionable cable bolt between the mechanical anchor/resin and the first end of the tensionable cable bolt. When installation of the tensionable cable bolt is complete and torque from a bolt installation machine is removed, the twisted portion of the tensionable cable bolt can untwist, which causes the tension applied to the tensionable cable bolt to be reduced. To counteract tensional deformation, a sleeve or buttons may be fixed to the portion of the tensional cable bolt susceptible to torsional deformation. However, these additional components can add to the cost of manufacturing a tensionable cable bolt.
Another problem related to tensionable cable bolts is that it is often difficult to tell whether or not the tensionable cable bolt has been properly tensioned. If the tensionable cable bolt is not tensioned properly, it will not adequately support a mine roof. Causes of improper tensioning include the torsional deformation discussed earlier, inadequately mixed resin and adhesive, or non-gripping or non-deployment of the mechanical anchor. However, each of these problems occurs inside the bore hole and are, therefore, obscured from view.
Hence, a need remains for a mine roof cable bolt which resists torsional deformation during installation with subsequent loss of tension, increases resin mixing time, and provides an affirmative visual indication of proper tensioning.
It is therefore an object of the present invention to provide a cable bolt that resists torsional deformation, delays tensioning to increase mixing time, and provides a visual indication that the cable bolt is properly tensioned.
In general, the present invention includes a mine roof support device having an elongated body, such as a multi-strand cable segment, having a first end, a second end, and forming an exterior surface. A mechanical anchor may be positioned between the first end and the second end of the elongated body, a drive head may be positioned adjacent to the first end of the elongated body, and a mixing delay device may be positioned between the drive head and the mechanical anchor.
The mixing delay device is configured to withstand a predetermined amount of an externally applied compression force, and then compress when the externally applied compression force exceeds the resisting force. Once compressed, the mixing delay device continues to exert the resisting force against the externally applied compression force. Suitable mixing delay devices include a lock washer, a Belleville type of washer, or other suitable device.
A flat washer may be positioned between the mixing delay device and the second end of the elongated body, a bearing plate may be positioned between the mixing delay device and the mechanical anchor, a barrel and wedge assembly may be positioned between the drive head and the mixing delay device, a stiffening sleeve defining a hollow cavity configured to receive the elongated member may be positioned adjacent to the barrel and wedge assembly, and a material coating, forming an optional textured surface, may be positioned on the exterior surface of the elongated body. A button may be positioned between the first end and the second end of the elongated body.
One method of supporting a mine roof is also included. The method generally includes the steps of a) drilling a bore hole in a mine roof, wherein the mine roof defines a wall surrounding the bore hole; b) inserting resin in the bore hole; c) providing an elongated body comprising a first end, a second end, a mechanical anchor positioned between the first end and the second end, a drive head positioned adjacent to the first end, and a mixing delay device positioned between the mechanical anchor and the drive head; d) inserting the second end of the elongated body into the bore hole; and e) rotating the elongated body in the bore hole. Additional steps may include f) engaging the mechanical anchor with the wall surrounding the bore hole after the step of rotating the elongated body in the bore hole; g) mixing the resin in the bore hole after the step of rotating the elongated body in the bore hole; h) advancing the elongated body into the bore hole after the step of rotating the elongated body in the bore hole; i) delaying the advancement of the elongated body into the bore hole after the step of rotating the elongated body in the bore hole; j) compressing the mixing delay device with a compression force after the step of delaying the advancement of the elongated body into the bore hole; and k) inspecting the mixing delay device after the step of compressing the mixing delay device with a compression force.
As stated earlier, the present invention helps an elongated body such as a multi-strand cable segment resist torsional deformation during installation, increases resin mixing time, and provides an affirmative visual indication of proper tensioning.
These and other advantages of the present invention will be clarified in the Detailed Description of the Preferred Embodiments and the attached figures in which like reference numerals represent like elements throughout.